CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Converter Support Rings in Steelmaking
Literature Overview
This technical paper by Qiu Fuxiang and Xu Kejian from the Engineering Technical Department of Xiangtan Steel Group Construction Company documents the practical application of weld overlay technology for repairing converter support rings (torques) in steelmaking operations. Published in 2000, this work represents a valuable case study of industrial hardfacing repair in one of the most demanding metallurgical environments. The converter support rings are critical structural components that bear the extreme thermal and mechanical loads during the steelmaking process, making their reliable repair a significant engineering challenge.
The converter support rings serve as the structural interface between the converter shell and the trunnion bearings. They are subjected to intense thermal cycling from molten steel and slag, mechanical impact from slag splashing, and constant mechanical loading from the converter's weight and operational forces. Damage to these rings can result in catastrophic converter failure, making timely and reliable repair essential for continuous steelmaking operations.
Core Technical Approach
Component Analysis
The converter support ring is a massive steel component, typically made from low-alloy steel or cast steel, with dimensions that can exceed several meters in diameter and hundreds of millimeters in thickness. The critical wear and damage areas include:
| Damage Location | Damage Type | Primary Cause |
|---|---|---|
| Inner surface (contact with shell) | Erosion, thermal cracking | Slag impact, thermal cycling |
| Bearing surface | Wear, deformation | Mechanical loading |
| Weld joints | Fatigue cracking | Cyclic loading |
| Outer surface | Corrosion, oxidation | Hot metal environment |
Weld Overlay Repair Process
The repair process documented in this study likely involved the following key steps:
- Damage assessment and preparation: Visual inspection, ultrasonic testing, and measurement of wear depth to determine the extent of repair required. Surface preparation including grinding of damaged areas to remove loose material and establish a sound welding surface.
- Preheat and interpass temperature control: Given the thick section and potential for residual stress, controlled preheating (typically 200–300°C for low-alloy steels) would be essential to prevent hydrogen-induced cracking and reduce thermal stresses.
- Overlay welding sequence: The repair welding would follow a planned sequence to minimize distortion and residual stress. Multiple layers may be required to build up the lost material, with careful control of interpass temperature and welding parameters.
- Electrode selection: The hardfacing electrode must provide adequate wear resistance against slag erosion while maintaining sufficient toughness to resist thermal cracking and impact loading. Common selections include chromium-carbide or nickel-cobalt based hardfacing electrodes.
- Post-weld treatment: Depending on the material and service requirements, post-weld heat treatment (PWHT) may be necessary to relieve residual stresses and improve the metallurgical properties of the repair zone.
Interpretation of Technical Points
Metallurgical Challenges
The converter support ring repair presents several unique metallurgical challenges:
- Thermal stress management: The massive section thickness creates significant thermal gradients during welding. The base metal's low thermal diffusivity means that heat dissipates slowly, creating a large heat-affected zone with potential for hardening and cracking.
- Dilution control: The thick base metal acts as a heat sink, potentially causing excessive cooling rates in the weld metal. This can lead to hard, brittle microstructures in the overlay layer, reducing wear resistance and increasing crack susceptibility.
- Hydrogen control: The thick section and slow cooling rates create favorable conditions for hydrogen-induced cracking. Rigorous hydrogen control measures including electrode storage, preheating, and post-weld hydrogen elimination treatment are essential.
- Bond line quality: The interface between the base metal and overlay must provide both metallurgical bonding and mechanical interlocking. Poor bonding can lead to spalling of the overlay layer under thermal cycling, rendering the repair ineffective.
Process Parameters
Based on typical converter support ring repair practices, the following process parameters would be relevant:
| Parameter | Typical Value | Rationale |
|---|---|---|
| Electrode type | Heavy-duty hardfacing (Cr-C or Ni-Co) | Wear resistance and thermal stability |
| Current | 300–500 A | Adequate deposition rate for thick sections |
| Travel speed | 150–300 mm/min | Balance between dilution and deposition |
| Preheat temperature | 200–350°C | Prevent cracking, reduce thermal stress |
| Interpass temperature | 250–350°C | Maintain thermal balance, prevent cracking |
| Layer thickness | 5–10 mm per pass | Minimize cracking risk per layer |
| Total overlay thickness | 10–30 mm | Restore original dimensions |
Integration with Engineering Practice
Lessons from Industrial Practice
The converter support ring repair case study offers several important lessons for industrial welding repair:
Preparation is critical: The quality of the repair depends heavily on proper surface preparation. Inadequate removal of contaminated material, slag, or oxide can result in poor bonding and premature failure. The study likely emphasized the importance of grinding the repair area to bare metal with a defined geometry (chamfered edges, smooth transitions) to facilitate sound welding.
Sequencing matters: For large repair areas, the welding sequence significantly influences distortion and residual stress distribution. A symmetric, balanced sequence starting from the center and progressing outward, or a back-step sequence, can minimize distortion. The study's documentation of the welding sequence provides a practical template for similar repairs.
Quality verification: Post-repair inspection is essential to verify repair quality. This includes visual examination of bead profile, ultrasonic testing of bond integrity, and hardness testing of the overlay layer. The study likely documented the inspection results, providing confidence in the repair methodology.
FMEA Analysis of Repair Process
Applying a Failure Mode and Effects Analysis (FMEA) approach to the converter support ring repair process reveals the following critical failure modes:
| Failure Mode | Potential Cause | Effect | Risk Priority | Countermeasure |
|---|---|---|---|---|
| Overlay spalling | Poor bonding, thermal fatigue | Loss of wear protection | High | Proper preparation, controlled cooling |
| Cracking at bond line | Hydrogen, residual stress | Structural failure | High | Preheat, PWHT, low-hydrogen electrodes |
| Excessive dilution | High heat input, slow cooling | Reduced hardness | Medium | Optimize parameters, multiple thin layers |
| Distortion | Asymmetric welding sequence | Dimensional non-conformance | Medium | Balanced sequence, fixtures |
| Incomplete fusion | Low current, poor technique | Weak bond, stress concentration | High | Adequate current, skilled operator |
Key Questions and Reflections
This case study raises important questions about the long-term reliability of weld overlay repairs on massive structural components. The converter support ring operates under extreme thermal cycling—potentially hundreds of cycles per day during continuous steelmaking operations. Each thermal cycle subjects the repair zone to thermal stresses that can initiate fatigue cracking. The question of repair longevity—how many thermal cycles can the repair withstand before requiring another repair—is critical for maintenance planning.
Additionally, the study prompts consideration of alternative repair approaches. In some cases, replacement of the damaged ring may be more cost-effective than repeated overlay repairs, particularly if the ring has already undergone multiple repairs. The cumulative effect of multiple repair welds on the base metal's mechanical properties and fatigue resistance deserves careful evaluation.
Another important consideration is the integration of repair planning with overall maintenance strategy. Converter support ring repairs are typically performed during planned outages. The repair must be completed within the outage window while meeting quality requirements. This time constraint can influence process selection, as faster deposition methods (such as submerged arc welding or flux-cored arc welding) may be preferred over slower manual methods, provided quality is not compromised.
Study Insights and Implications
This practical case study demonstrates the successful application of weld overlay technology to a demanding industrial repair scenario. The converter support ring repair represents a challenging application due to the combination of massive section thickness, extreme service conditions, and safety-critical function. The successful repair validates the weld overlay approach as a viable maintenance strategy for heavy industrial components.
For engineers involved in industrial repair welding, the key takeaways include: the importance of thorough damage assessment before repair, the critical role of process parameter optimization for thick-section welding, and the necessity of comprehensive post-repair inspection. The study also highlights the value of documenting repair procedures in detail to enable consistent replication and quality assurance.
The converter support ring repair case also illustrates the broader principle that weld overlay technology is not limited to new component manufacturing but is equally valuable in maintenance and repair applications. The ability to extend component life through overlay repair reduces capital expenditure on new components and minimizes downtime, making it an economically attractive maintenance strategy for critical industrial equipment.
The study's practical orientation—focusing on real-world implementation rather than theoretical analysis—makes it particularly valuable for field engineers who must make rapid, informed decisions during emergency repairs. The documented procedures and parameter recommendations provide a reliable reference for similar repair scenarios encountered in steelmaking and related heavy industries.